Carbohydrates

Carbohydrates are the most abundant macromolecules on this earth. They comprise of carbon dioxide, water, and glucose. Photosynthesis converts carbon dioxide and water into glucose and starch, both of which are carbohydrates. This is how all of sorts of carbohydrates originate on the earth.

Every year, nearly 100 to 115 billion tons of carbon is converted into biomass by photosynthetic organisms worldwide. This means 100 to 115 billion tons of carbohydrates are produced per year. 

Carbohydrates Definition

Carbohydrates are central to nutrition, and we find them in a wide variety of natural and processed foods. They perform various functions in living organisms, from yielding energy to participating in structure formation to cellular recognitions and adhesions.

What are carbohydrates? – Nutritionally speaking, carbohydrates or carbs are the most important component of our daily food. Like sugar and starch, certain carbohydrates form the dietary staples in many countries worldwide. A wide variety of food items are available as sources of carbohydrates.

Biochemically speaking, carbohydrates are macromolecules made of carbon, hydrogen, and oxygen. The ratio of hydrogen: oxygen is usually 2:1, just like in water. One can write the empirical formula of carbohydrates as C_m(H₂O)n, where m and n may be the same or different. The term ‘saccharide’ is more commonly used in biochemistry to denote carbohydrates and different prefixes are added to refer to their specific types.

What are the Different Types of Carbohydrates?

Carbohydrates are classified into three types:

• Sugars
• Fibre
• Starches

Sugars: Also known as simple carbs as they are in their basic form. They are found in desserts, processed foods, and ordinary soda. They also occur naturally in fruits, vegetables, and milk.

Starches: Mainly called complex carbohydrates made up of several simple sugars connected. Our body breaks carbohydrates into sugars to convert them to energy. Examples of starches are: bread, pasta, cereals, etc. Vegetables like corn, potatoes are also an example of starches. 

Fibre: They are also a kind of complex carbohydrate. As most fibres cannot be broken down by the body, having fibre-rich meals will help our body provide additional health benefits. Fibres help in reducing cholesterol and blood sugar levels. Fibre is found in various plant-based foods, fruits, vegetables, nuts, beans, and whole grains.

Sources of Carbohydrates

Nearly three-fourths of the biomass in plants is made up of carbohydrates. Hence, plants and plant products are the most abundant sources of carbohydrates. Animals have these ‘saccharides’ in comparatively much fewer quantities, and the most common forms of animal carbohydrates are glycogen, sugars, and their derivatives.

Carbohydrate-rich food items are:

1. Plant-based products such as grains, fruits like banana, sapota, pineapple, grapes, vegetables such as potato, sweet potato, beetroot, beans, tapioca, etc., are the raw materials for making table sugar.
   
2. Animal-based products such as honey, milk and milk products. 

Sources of carbohydrates can also be classified as healthy and unhealthy sources. 

Healthy Sources of Carbohydrates	Unhealthy Sources of Carbohydrates
Unprocessed or minimally processed foods	Processed foods and food items rich in sugar content
Carbohydrates examples: whole grains, fruits, vegetables, and beans.	Examples: white bread, pastries, cakes, cold drinks, sweetened juices, refined flour, etc.

Classification of Carbohydrates

Carbohydrates can be classified into two:
(i) Nutritional classification
(ii) Structural classification.

Nutritional Classification of Carbohydrates

Different types of carbohydrates contribute to our nutrition at different levels:

1. Simple carbohydrates: These are easiest to digest and provide quick energy. Fruits like banana, pineapple, berries, grapes, oranges, and other items like honey, glucose, and milk are sources of simple carbs. 
2. Starchy carbohydrates: These are a little more complex than simple carbs and take longer to digest. Starchy carbs are grains like wheat, rice, jowar, vegetables such as potato, sweet potato, etc.
3. Fibrous carbohydrates: Dietary fibers are fibrous carbohydrates, also known as roughage.  These fibrous carbs take very long to convert to simple sugars through digestion. They slow down the conversion of glucose to glycogen and the fibers are not digested by human enzymes. They also contribute significantly towards bulking up undigested waste materials and maintaining the gut’s healthy bacteria. Green leafy vegetables, pumpkin, carrots, tomatoes, cabbage, broccoli, beans are some sources of fibrous carbohydrates. Grains like oats are also rich in dietary fibers. 
4. Complex carbohydrates: These are complex as the name suggests. Food items like whole grains, pasta, brown rice, red rice, and oats are a few examples of complex carbs. 

Carbohydrates Structure

Chemically carbohydrates are polyhydroxy aldehydes or ketones. These are the chemical substances obtained upon hydrolysis of carbohydrates. The empirical formula of carbohydrates can be written as (CH₂O)_n. We have seen that basic carbohydrates elements: carbon, hydrogen, and oxygen, but some even contain nitrogen, phosphorus, and sulphur. Further, carbs exist as polymers of simple units (‘saccharon’). The number of simple units and their polymerization differs in different types of carbohydrates. Based on the presence of carbohydrates monomer, the carbs are classified as:

1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides

1. Monosaccharides: These are referred to as simple sugars. Chemically, they can be aldoses and ketoses with two or more hydroxyl groups. The backbone of the most common monosaccharides is an unbranched carbon chain where a single bond joins all carbon atoms. One carbonyl group is attached to the terminal carbon atom. If this carbonyl group is an aldehyde, then that type of monosaccharide is known as aldose. If the carbonyl group is a ketone, the monosaccharide type becomes ketose.

Depending on the number of carbon atoms in the monosaccharides, they are classified as:

a. Triose: number of carbon atoms in these monosaccharides is 3 and the two simplest trioses are glyceraldehyde and aldotriose. 

b. Tetrose: when the number of carbon atoms in the backbone chain are 4, such a monosaccharide becomes tetrose. Example: erythrose.

c. Pentose: if the carbon atoms in the backbone chain are 5, the monosaccharide is referred to as pentose. Examples: ribose and deoxyribose.

d. Hexose: when 6 carbon atoms are present in the backbone chain of a monosaccharide, it is known as hexose. Examples: glucose and fructose

e. Heptose: monosaccharides with 7 carbon atoms in their backbone chains are known as heptoses. Example: sedoheptulose.
Thus, a monosaccharide can be aldotriose or ketotriose, aldohexose or ketohexose and so on. 

2. Disaccharides: When a carbohydrate contains two units of monosaccharides, it is called a disaccharide. The two monosaccharide units are joined covalently with an O-glycosidic bond. Common examples of disaccharides are maltose, sucrose, and lactose. Maltose has two glucose units; lactose has one unit of glucose and one unit of galactose whereas sucrose has glucose units and fructose. Sucrose is the common table sugar.

3. Oligosaccharides: These are carbs with three to ten monomeric units and are thus short polymers. Few rich sources of plant oligosaccharides are burdock, chicory, Jerusalem artichoke, onions, celery, etc.

4. Polysaccharides: These are polymers where more than ten monosaccharide units are linked with each other. They are commonly called glycans and are high molecular weight macromolecules. They differ like the monomeric units, length of their chains, types of bonds linking the chains, and degree of branching. Homopolysaccharides have a single monomer whereas heteropolysaccharides have two or more different types of monomers. Starch,  glycogen, cellulose, chitin are examples of homopolysaccharides. Bacterial cell envelopes and extracellular matrices in animals have many heteropolysaccharides in them.

Types of Polysaccharides

Some important polysaccharides:

Carbohydrates Function

Carbohydrates play an important role in the body and perform several functions. Some of the functions of carbs are:

1. Energy production: Carbohydrates provide energy to the body. Except for dietary fibres, other types of carbs get digested and converted into glucose. Each cell takes up glucose from the blood and it gets broken down further in cellular respiration, releasing energy in the form of ATP. These ATP molecules are then used for various biological functions.
2. Energy storage: Carbohydrates provide stored energy. Excess glucose converted into glycogen and gets stored in the liver and muscles. Glycogen gets converted into glucose to provide energy and thus helps maintain a constant blood glucose level. Glycogen also acts as a reserve of energy. In plants, excess glucose gets converted into starch and is stored in various parts.
3. Promoting digestive health: Unlike starch and glucose, dietary fibres are not broken down into glucose. They remain undigested. The dietary fibres make the bulk of the stools and help bowel movements.
4. Building macromolecules: Most of the absorbed glucose is used for energy production. Some amount is converted into ribose and deoxyribose. These sugars are the building blocks of other macromolecules such as DNA, RNA and ATP. Glucose is also utilized to make NADPH, which acts as an antioxidant.
5. Role in lipid metabolism: A rise in blood glucose level prevents lipid usage by the body for energy production.  Thus, it prevents fat loss and ketosis in the body.
6. Role as flavoring agent and sweeteners: Carbs are naturally sweet. They are used as sweeteners and flavour enhancers in various food products.

Deficiency of Carbohydrates

The abundance of carbs in nature and the capacity of the human body to store extra carbs make the carbohydrate deficiency rare. However, diets low in carbs lead to moderate deficiency and may be various factors such as diabetic ketosis, low-carb diets, etc. A carbohydrate-deficient diet shows headaches, nausea, fatigue, weakness, difficulty in concentrating, bad breath, constipation, and vitamin and mineral deficiencies.

Prolonged Deficiency of Carbohydrates leads to the following ailments or health issues:

1. Acidosis: The body shifts from carbohydrates to lipids for energy production, thus increasing lipolysis. Increased lipolysis leads to increased production of ketoacids. Ketoacids are responsible for increased acidity of the blood and other body fluids, which, in turn, cause harmful and irreversible damage to body tissues.
2. Ketosis: During prolonged carbohydrate starvation, the body breaks down fatty acids and amino acids for energy production. During this breakdown, it produces ketone bodies. Excessive formation of ketone bodies leads to ketosis, which has harmful effects on the body.
3. Hypoglycemia: Deficiency of carbs leads to low blood glucose level, a condition known as hypoglycemia. Symptoms of hypoglycemia are dizziness, fatigue, distress, and delirium.
4. Constipation: Dietary fibres are an important component of our daily foods and they help bulk up the stools. The absence of dietary fibres may lead to severe constipation.

Related Topics to Study

1. Fibre
2. Importance of Carbohydrates
3. Food Variety

Summary

In our bodies, carbohydrates are those important macromolecules primarily utilized for energy production. Carbohydrates are naturally sweet. They are used as sweeteners and flavour enhancers in several food products. Good carbohydrates are very important for a healthy body, and eating bad carbohydrates leads to various diseases. So, watch out for what you are eating!

FAQs on Carbohydrates

Following are the frequently asked questions on carbohydrates:

Q.1. What are the 3 main carbohydrates?
Ans: Structurally, carbohydrates have three main types: monosaccharides, disaccharides, and polysaccharides. 

Q.2. What is a carbohydrate?
Ans: Chemically carbohydrates are polyhydroxy aldehydes or ketones. Nutritionally, they are macromolecules and macronutrients abundantly found in grains, fruits, and vegetables.

Q3. Which foods have carbohydrates?
Ans: Carbohydrates foods are whole grains, fruits, vegetables, honey, milk, and milk products are some richest sources of carbohydrate food items.

Q.4 What are bad carbs to eat?
Ans: Nutritionally, bad carbs are those which are easy to digest but lead to various diseases such as obesity, increased cholesterol levels, diabetes, heart problems, etc. White bread, pastries, and cold drinks are a few sources of bad carbohydrates

Q.5. Is rice a carbohydrate?
Ans: Yes, rice is a rich source of carbs. Starchy carbohydrates are plentiful in all types of rice.